it means to place your tongue between your lips and blow.Blowing raspberries is usually used as an insult and way of taunting or a gesture of disrespect .If somebody blows raspberries at another guy he is mocking him and it`s like telling him he is a loser and a sucker.In many cartoons you can see a character who blows raspberries at other characters and this is how he mocks them and taunts them.The little children are also taunting blowing raspberries at each other.The blowing raspberries insult is very similar to the British taunting called cocking a snook in which the thumb of one hand is on the nose and the extended fingers are wiggled and the tongue is sticked out.

Examples of blowing raspberries
Example 1:

A couple is splitting up.

The Girl: Please don`t leave me.

The Boy: Get the fuck out.

The Girl: But please I`ll do anything to make the things go right between us.

The Boy: Get the fuck out we are over I don`t wanna ever see you again *turns his back on the girl an leaves*

The Girl: *blows raspberries at the boy and goes away*
Example 2:

A boy is trying to catch a girl and beat her up for taunting him.The girl escapes the boy and quickly enters a house and locks the door so the boy is unable to catch her.Then the girl shows up in a windows and says:"Nanner,nanner,nanner, you can`t catch me" and then blows raspberries at the boy.